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Programmable ROMs

Programmable Read Only Memory and Erasable Programmable Read Only Memory (EPROM) are the programmable logic devices in which AND array is fixed and OR array is programmable. The programmable ROM implements the arbitrary combinational function. Further, programmable ROM are used as a memory storage device having n inputs of used as address lines and m outputs used as data lines. Here, the AND array produces all possible minterms of a input variables, and the programmable OR array
allows minterms to appear at the inputs. Figure below shows the internal architecture of a PROM having four input lines, a hard-wired array of 16 AND gates and a programmable array
of four OR gates.

EPROMs and Electrically Erasable Programmable Read Only Memory (EEPROMs) are programmed through the PROM programmers.

Disadvantages of PROM

Â inefficient use of logic capacity.

It is costly to use PROMs for applications where lower minterms are needed.